How to deal with appliances that will need to be replaced.

[Eric Bradeis]

So I have my first deal under contract and it's a tri-plex. After the inspection, it is brought to my attention that the one of the furnaces, one of the water boilers, and a condenser unit for the AC are all really old and past their prime. They are all in working condition currently. I plan on saving 5-7% of the monthly rent for capital expenditures. But I just want to be more prepared. When do u replace these items? Do u wait until they crap out to buy a new one? Do u buy a new one while the old one is still working? Won't it cost way more if u wait until it totally craps out? I'd appreciate any feedback or advice. Thx.